A hot work tool is required to have sufficient toughness to resistant to impact since the tool is used in contact with high-temperature or hard workpieces. Conventionally, alloy tool steel, such as SKD61 of JIS steel grade, has been used for a hot work tool material. Recently, further improved toughness has been required and thus an alloy tool steel having modified composition of the SKD61 alloy tool steel has been proposed (see Patent Literatures 1 to 3).
A material for the hot work tool is typically manufactured from a raw material steel piece, as a starting material, such as of a steel ingot or a bloom which is bloomed from the steel ingot. The starting material is subjected to various hot working and heat treatment to produce a predetermined steel material, and the steel material is finished by annealing. A hot work tool material in the annealed condition having a low hardness is typically supplied to a manufacturer of the hot work tool. The supplied material is machined into a shape of the hot work tool and then quenched and tempered to adjust its hardness for use. After the adjustment of the hardness, finishing machining is typically conducted. In some cases, quenching and tempering are conducted first for the material in the annealed condition, and then the machining is conducted for the shaping of the tool together with the finishing machining. Here, the term “quenching” refers to an operation where a hot work tool material (or a hot work tool material that has been subjected to machining) is heated to an austenitic phase temperature range and then rapidly cooled to transform it into a martensitic structure. Thus, the hot work tool material has such a composition that can have a martensitic structure by the quenching.
In this connection, it has been known that a toughness of the hot work tool can be improved by properly controlling an annealed structure prior to quenching and tempering of the hot work tool material. For example, proposed is a hot work tool material having an annealed structure including uniformly dispersed carbides therein, since precipitation of acicular carbides along a coarse bainite grain boundaries is suppressed by annealing the steel material in which precipitation of coarse bainite is suppressed (see Patent Literature 4). A hot work tool material having excellent toughness can be obtained when the material including uniformly dispersed carbides is quenched and tempered.